I haven't had anyone inquire about it before, but if I was to charge for it, I would charge according to the time I use to do it. Considering they would probably need to reapply some of the gel polish (if you have that) because filing around the nail to change the shape would break the top seal, and chip the gel polish. So it would likely not be a case of "just" changing the shape. They might have to buff the gel polish and reapply another thin coat + new top coat too, and if there is any nail art on there, it might affect it... might not be as cheap as you want it to be.
The best you can do is phone and ask for a quote though.